This invention relates generally to a circuit which involves phase shifting and specifically to a television reception circuit suitable to receive a broadcasting channel selected from among a plurality of channels and output a signal of the received broadcasting channel as an intermediate frequency signal, and more particularly to an integrated television reception circuit which can efficiently remove an image frequency. The preasent invention relates also to a phase shifting circuit for shifting the phase of a signal inputted thereto, and more particularly to an automatic phase shift control circuit which can set a phase shift amount accurately within a broad frequency range. The present invention further relates to an equal amplitude addition circuit which adds signals inputted thereto with amplitudes equal to each other, and more particularly to an equal amplitude addition circuit suitable to add two balanced input signals with amplitudes equal to each other.
A reception circuit for selecting and receiving a desired channel from among a large number of television broadcasting channels includes a radio frequency amplifier for amplifying the selected broadcasting signal and a frequency mixing circuit for converting the radio frequency output of the radio frequency amplifier into an intermediate frequency, and has such a circuit construction as shown, for example, in FIG. 17.
Referring to FIG. 17, the reception circuit shown includes a filter 1 of the single-tuned type for selecting a desired frequency from among channel frequencies of broadcasting radio waves received by an antenna, and an amplifier 2 of the automatic gain control type for controlling the level of the received signal selected by the filter 1. The amplifier 2 is controlled based on a demodulation output (AGC) not shown so that the output level thereof may remain within a predetermined level range.
The reception circuit further includes a double-tuned filter 3 for limiting the pass band of the television signal. The double-tuned filter 3 cooperates with the single-tuned filter to vary the pass frequency of the broadcasting radio wave of the selected channel in accordance with a channel selection signal not shown.
The reception circuit further includes a trap circuit 4 for removing from within the received broadcasting frequency an image frequency which is mixed into the received broadcasting frequency when the broadcasting frequency is converted into an intermediate frequency. As hereinafter described, the frequency to be removed is set by the trap circuit 4 so that an image intermediate frequency which serves as a disturbing wave may not be produced. The reception circuit further includes a frequency conversion circuit (mixing circuit) 5 for converting the inputted radio frequency signal into an intermediate frequency (58.75 MHz) signal. The frequency conversion circuit 5 is formed from an integrated circuit which includes a local signal oscillator Lo, a frequency mixer MIX, and an intermediate frequency amplifier IFA for selecting an intermediate frequency.
In the television reception circuit described above, a local oscillation frequency (which may be hereinafter referred to as local frequency) fLo is set corresponding to a frequency (which may be hereinafter referred as selected frequency) fD of a desired selected channel as seen from FIG. 18, and the local frequency fLo and the selected frequency fD are mixed by the frequency mixer MIX so that a frequency of a difference between them is outputted as an intermediate frequency fIF from the frequency mixer MIX.
By the way, if an image frequency fIM is mixed at a point spaced by the intermediate frequency fIF from the local frequency fLo in the frequency mixer MIX, then the frequency mixer MIX outputs an undesired intermediate frequency fIF(IM) which includes the same frequency component also with regard to the image frequency fIM. Therefore, in order to prevent the image frequency component fIM, which makes an image diturbance, from being mixed in the frequency mixer MIX, the television reception circuit of FIG. 17 includes the trap circuit 4 for removing the image frequency.
Consequently, the television reception circuit described above is disadvantageous in that the channel selection control circuit thereof is complicated because the three circuits of the single-tuned circuit, double-tuned circuit and image trap circuit must be adjusted each time the selected channel is changed and also in that it is difficult to adjust the channel selection control circuit so that an image disturbance may be reduced in an entire reception band.
Thus, another reception circuit has been proposed wherein a mixing circuit formed as an integrated circuit includes two frequency mixers to cancel an image frequency.
FIG. 19 shows a construction in principle of the mixing circuit just mentioned. Referring to FIG. 19, a frequency fD of a desired selected channel is supplied to a first frequency mixer MIX(Q) and a second frequency mixer MIX(I). Meanwhile, local frequencies fLo(Q) and fLo(I) produced by a xcfx80/2 phase shifter PH and having a phase difference of 90 degrees from each other are supplied to the frequency mixers MIX(Q) and MIX(I), respectively, and intermediate frequencies corresponding to frequency differences between the selected channel frequency and the local frequencies are outputted from the first and second frequency mixers MIX(Q) and MIX(I), respectively. The intermediate frequencies are outputted through a phase shifter PSN by which they are shifted by xcfx80/2 relative to each other, and then combined by an adder ADD, thereby cancelling an inputted intermediate frequency fIM (refer to IEEE Transactions on Consumer Electronics, Vol. 38, No. 3, August, 1992).
However, while the image cancelling mixing circuit described above has been put into practical use in reception circuits for radio frequencies included in an ordinary comparatively low frequency band, it is difficult to use it for a television reception circuit.
The reason is that, with regard to a broadcasting signal having a frequency band of several MHz as in the case of television broadcasting waves, it is difficult to form the xcfx80/2 phase shifter PH of the image cancelling mixing circuit so that it may shift the phases of local oscillation frequencies accurately by xcfx80/2 whichever broadcasting channel is received because of an influence of a dispersion of resistance, a dispersion of capacitance and parasitic floating capacitance when an integrated circuit is formed. Further, a high cost is required because a large number of elements are involved. From those reasons, it is considered difficult to put the image cancelling mixing circuit described above into practical use.
A phase shifting circuit which is employed in such an image cancelling mixing circuit as described above and causes the phase of an ac signal to lead or lag has been realized using various circuits. Where a phase shifting circuit is formed, for example, from an analog circuit in which a circuit device formed from resistors and capacitors is used, it is difficult to form the phase shifting circuit so that it may exhibit a fixed phase shift amount over a broad frequency range.
This arises from the fact that, with a circuit wherein capacitors having impedance and resistors are integrated, it is difficult to set a phase shift amount having a high degree of accuracy because of an absolute dispersion in the impedance or resistance and the fact that the phase shift amount is varied by an influence of a floating capacitance and so forth of the circuit by the frequency of an ac signal inputted to the phase shifting circuit. It is difficult to form a phase shifting circuit of the type described so that it shifts the phase of an input signal by a fixed angle over a broad frequency range.
Particularly where a phase shifting circuit is formed as an integrated circuit, dispersions in resistance of the circuit and floating capacitance derived in the circuit make it difficult to provide a desired phase shift amount to an inputted signal. Further, in such a circuit that an input signal exhibits a variation in frequency, it is difficult to always provide an accurate phase shift amount even if resistors, capacitors and so forth which form a phase shifting circuit are connected externally.
In the meantime, an ordinary signal addition circuit is constructed such that signals of given levels are inputted as they are to an adder.
An addition circuit of such an ordinary type as just mentioned outputs, if it adds a signal whose level is xe2x80x9caxe2x80x9d and another signal whose level is xe2x80x9cbxe2x80x9d, a signal whose level is xe2x80x9ca+bxe2x80x9d.
Meanwhile, a circuit of the type which performs addition to remove an unnecessary signal component such as, for example, the image frequency component fIM in the television reception circuit described hereinabove with reference to FIG. 17 is required to add two inputted signals with equal levels to each other. However, an ordinary addition circuit provides an addition output of a varying level where the levels of inputted signals to be added do not have a fixed value.
Particularly where signals having a phase difference are combined with each other, it is sometimes demanded to add two signals having different amplitudes so that the levels thereof may be equal to each other. However, the demand cannot be satisfied by an ordinary addition circuit. Particularly where an addition circuit for adding signals in a radio frequency region is formed as an integrated circuit, it is difficult to form an addition circuit having a high degree of accuracy because of an influence of dispersions of resistance values and/or floating capacitances.
It is an object of the present invention to provide a television reception circuit wherein a front end circuit of a television receiver is integrated and outputting of an image frequency is suppressed.
It is another object of the present invention to provide a phase shift control circuit wherein a phase shift amount of an inputted ac signal can be set accurately.
It is a further object of the present invention to provide an equal amplitude addition circuit wherein signals of arbitrary levels inputted can be added with levels equal to each other.
In order to attain the objects described above, according to an aspect of the present invention, there is provided a television signal reception circuit, comprising an integrated circuit including an automatic gain circuit for controlling a level of an input signal, first and second frequency mixture circuits to which an output of the automatic gain circuit is inputted, a xcfx80/2 phase shifting circuit for supplying local oscillation frequencies having a phase difference of xcfx80/2 from each other to the first and second frequency mixture circuits, a first xcfx80/4 phase shifting circuit for shifting a phase of an output of the first frequency mixture circuit by +xcfx80/4, a second xcfx80/4 phase shifting circuit for shifting a phase of an output of the second frequency mixture circuit by xe2x88x92xcfx80/4, and an addition circuit for adding outputs of the first and second xcfx80/4 phase shifting circuits, the television signal reception circuit outputting a television signal inputted to the integrated circuit through a variable frequency tuning circuit as an intermediate frequency signal.
With the television signal reception circuit, since a signal processing circuit formed as the integrated circuit for converting a received television signal into an intermediate frequency signal is provided on a semiconductor integrated circuit board, absolute dispersions of the elements which compose the circuit are eliminated, and besides, since the reception circuit adopts a circuit of the image cancellation type formed from the components described above, miniaturization of the reception circuit can be achieved and a disturbing image frequency can be removed with a high degree of accuracy. Consequently, a trap circuit which is normally required for a channel selection circuit can be eliminated.
Preferably, the xcfx80/2 phase shifting circuit includes a phase comparator for comparing phases of phase-shifted outputs of the xcfx80/2 phase shifting circuit and signals prior to the phase shifting by the xcfx80/2 phase shifting circuit with each other, and a xcfx80/2 phase control circuit to which an output of the phase comparator is fed back. Here, the xcfx80/2 phase shifting circuit serves as an automatic phase shifting circuit which can effect accurate phase shifting over all frequencies. This reduces an influence of a dispersion of each element of the semiconductor integrated circuit. Consequently, adjustment operations required for the reception circuit are reduced remarkably, resulting in achievement in reduction of the cost and also in facilitation in designing.
Preferably, the addition circuit includes a level detector for detecting a level difference between first and second signals inputted thereto, and an adder for adding the first and second signals with levels equal to each other based on a detection output of the level detector. Here, the addition circuit serves as an automatic addition circuit which can effect complete removal of an image frequency. This reduces an influence of a dispersion of each element of the semiconductor integrated circuit. Consequently, adjustment operations required for the reception circuit are reduced remarkably, resulting in achievement in reduction of the cost and also in facilitation in designing.
According to another aspect of the present invention, there is provided an automatic phase shift control circuit, comprising a phase control circuit for variably controlling a phase shift amount in accordance with a control signal, a phase comparison circuit for comparing a phase difference between two output signals outputted from the phase control circuit and having predetermined phase amounts, a low frequency extraction circuit for extracting a dc component from a phase difference signal outputted from the phase comparison circuit, and a feedback circuit for feeding back an output of the low frequency extraction circuit as the control signal to the phase shift control circuit.
With the automatic phase shift control circuit, the phase shift amount of the phase control circuit can be corrected automatically so that it may be equal to a set phase shift amount. Consequently, even when the frequency of an input signal whose phase is to be shifted exhibits a great variation, the automatic phase shift control circuit can produce a phase shifted signal having a phase shifted accurately by a predetermined amount.
Further, also where the automatic phase shift control circuit is formed on a semiconductor integrated circuit board, absolute dispersions of the elements which form the circuit can be absorbed by the circuitry which feeds back the result of the phase comparison. Consequently, the automatic phase shift control circuit can be produced with a reduced cost using a comparatively small number of elements and adjustment operations therefor can be reduced remarkably, resulting in achievement in reduction of the cost.
Preferably, the phase shift control circuit is a xcfx80/2 phase shift control circuit. This makes the advantages of the automatic phase shift control circuit described above further prominent.
Preferably, the automatic phase shift control circuit is formed from an integrated circuit wherein analog circuits of the balance type are integrated. This can reduce a phase shift error which originates from an integrated circuit wiring pattern.
According to a further aspect of the present invention, there is provided an equal amplitude addition circuit, comprising a first differential amplification circuit for amplifying a first addition signal inputted thereto, a second differential amplification circuit for amplifying a second addition signal inputted thereto, first conversion means for converting an output of the first differential amplification circuit into a signal of a predetermined level in accordance with a control signal, second conversion means for converting an output of the second differential amplification circuit into a signal of a predetermined level in accordance with the control signal, detection means for detecting output levels of the first and second conversion means, gain control means for generating the control signal in response to a detection signal of the detection means, and addition means for adding signals outputted from the first and second conversion means, the levels of the output signals of the first and second conversion means being controlled so as to be equal to each other in accordance with the control signal outputted from the gain control means.
With the equal amplitude addition circuit, even when inputted signals have different levels, they are controlled so that they are added with levels equal to each other. Accordingly, the equal amplitude addition circuit exhibits a very high advantage particularly where it is applied to an addition circuit of the type which adds two signals to remove unnecessary signal components.
Further, also where the equal amplitude addition circuit is formed on a semiconductor integrated circuit board, absolute dispersions of the elements which form the circuit can be absorbed by the circuitry which feeds back the signal of the detected output level. Consequently, the equal amplitude addition circuit can be produced with a reduced cost using a comparatively small number of elements, and where the equal amplitude addition circuit is applied, for example, to a television reception circuit of the image cancellation type or the like, an image signal can be removed effectively and adjustment operations therefor can be reduced remarkably, resulting in achievement in reduction of the cost.
The first conversion means may include third and fourth differential pairs connected such that the output of the first differential amplification circuit may serve as common emitter current of the third and fourth differential pairs, and the second conversion means may include fifth and sixth differential pairs connected such that the output of the second differential amplification circuit may serve as common emitter current of the fifth and sixth differential pairs. In this instance, the detection means may detect the output levels of the first and second conversion means by detecting an output of the fourth differential pair and an output of the sixth differential pair.
The control signal may be used to control gains of the first and second conversion means in the opposite directions to each other. This simplifies a control circuit which uses the control signal.
Preferably, the first addition signal and the second addition signal have a phase difference of 90 degrees from each other.
Preferably, the equal amplitude addition circuit is formed from an integrated circuit wherein analog circuits of the balance type are integrated. This can reduce a phase shift error which originates from an integrated circuit wiring pattern.